Introduction

Spiders, along with daddy long legs, ticks, mites, and scorpions, belong to the class Arachnida. They are beneficial animals that feed on a variety of insects and other arthropods, including many we consider pests. About 3,000 species of spiders are found in the United States. Spiders rarely bite people, and most species found in the world are harmless. However, some people may be allergic to a spider’s bite, and a few species of spiders are known to produce bites that may have serious medical implications for humans. The most medically important spiders in the state are the black widow spider and the brown recluse spider and its relatives, which are rarely encountered in Pennsylvania. Black widows are native to Pennsylvania and generally found outdoors; brown recluse spiders are not native and cannot survive in Pennsylvania’s climate unless they remain inside heated structures. Many spiders indigenous to Pennsylvania will come into homes, mostly during the autumn, and a few of the more common and important of these will be discussed here.

This full-color publication discusses the more common and important spider species in Pennsylvania. Photographs are also provided.

A Note on Venom, “Toxic Venoms,” and “Poisonous Venoms”

All spiders have fangs and most possess venom. However, most spider bites have little or no effect on humans. The exceptions include people with compromised immune systems or other medical conditions that leave them vulnerable to spider venoms. Most spiders are too small to be of concern to humans. The fangs of these tiny creatures cannot penetrate the human epidermis unless the spider is held in place and allowed to bite the very thin skin between the fingers or toes. Larger spiders can bite, but they are shy and will attempt to escape unless trapped between the skin and some other object (e.g., clothing, sheets, and shoes).

Venoms from any spider are poisonous and/or toxic to their prey, but most are not poisonous or toxic to humans. The terms “poisonous venoms” and “toxic venoms” are somewhat misleading and should be replaced with “potentially harmful venoms.” Very few spiders produce venoms that are potentially harmful to humans.

Salticidae—Jumping Spiders

Sicariidae—Sixeyed Sicariid Spiders

Theridiidae—Cobweb Weavers

Control

Avoid bites by wearing gloves when doing yard work. Be careful when reaching under stones, logs, or firewood, or when reaching behind undisturbed household items such as cabinets, furniture, and boxes. Black widows, brown recluses, and other spiders tend to inhabit such undisturbed areas.

Reduce the number of potential nesting sites around the home. These include woodpiles, lumber stacks, rock piles, brush, high weeds and grasses, and discarded human-made items. Frequently cleaning and moving stored items in basements, storage areas, and garages will reduce the number of spiders by disturbing their habitats. Vacuum spiders and their webs from behind objects, under tables, and in wall and ceiling corners. Close openings in exterior walls and install weather stripping and thresholds at the bottom of doors. Leave firewood outside until you are ready to place it in a stove or fireplace. Firewood that is stored inside, even for short periods of time, will begin to warm, and any overwintering spiders hiding there will become active and may crawl out from under bark and crevices in the wood.

Some spiders will collect in large numbers on buildings with bright exterior lighting. The lights attract a variety of flying insects on which the spiders feed. Spider populations can be reduced by spraying a high-pressure water stream onto the building, and by switching from mercury vapor to sodium vapor exterior lighting.

Most insecticide sprays, whether they are applied to the interior or exterior of a building, do little to control or prevent spiders from entering. If spiders are sprayed with an insecticide, they will eventually die; however, it is still advisable to remove those spiders by the means previously mentioned rather than by applying a pesticide. If you have a confirmed infestation of either black widow or brown recluse spiders in your home, contact a licensed pest control company, the Penn State Department of Entomology, or the Penn State Extension office in your county for more information.

Glossary

Annulated—colored with darker banding and frequently referring to banded legs

Antivenom—a chemical antidote designed to counteract the effects of specific venom; also called antivenin

Ballooning—a behavior exhibited by some newborn arthropods wherein a recently hatched arthropod spins out some silk, which then catches the wind and carries the immature arthropod for a distance

Carapace—the hard integument forming the dorsal surface of the cepha-lothorax (not including the appendages)

Cephalothorax—the fused head/thorax region, as found in spiders

Chelicerae—(sing., “chelicera”) the front jaws of a spider consisting of a stout basal segment and a terminal fang

Chevron—a figure, pattern, or object having the shape of a “V” or an inverted “V”

Cytotoxin—any material that is destructive to cells

Diaphoresis—profuse perspiration

Distal—that portion of a structure that is farther from the central body (e.g., a human wrist is distal to the elbow)

Edema—excessive fluid buildup in cells or tissues

Envenomation—the introduction of venom into the body of another organism as a defense or feeding mechanism

Erythema—a flush on the skin surface produced by congestion within the capillaries

Femora (femur)—the third segment of the spider leg following the coxa and trochanter

Folium—a pattern or design on the abdomen surface

Millimeter—metric unit of length (25 millimeters = 1 inch)

Necrotic—tissue in a dead or decaying condition

Neurotoxin—any material that causes damage to the nervous system

Palps—(also “palpi” or “pedipalps”) paired, forward-projecting sensory organs of spiders located behind the chelicerae but in front of the legs; the second appendages of the cephalothorax

Papules—small, inflamed, congested areas of the skin

Patella—the fourth segment of the spider leg following the coax, trochanter, and femora (femur)

Penultimate instar—the next-to-last developmental stage of an arthropod before it molts into an adult

Procurved—eyes that are arranged in an upside-down “U” when viewed from the front

Pruritus—itching skin

Spinnerets—appendages that produce silk and are located at the posterior portion of the abdomen

Stabilimentum—a heavy band of silk deposited in the center of the web of some of the orb-weaving spiders

Tarsi—the last or terminal segments of a spider’s legs bearing two or occasionally three claws

Tibia—the fifth segment of the spider leg following the patella and preceding the tarsi

Venom—a poisonous substance that is produced by various animals (e.g., spiders, scorpions, and other arthropods; snakes and certain lizards) for defense or to subdue prey; can cause pain and swelling but rarely fatality when injected into humans

Venter—the underside of the spider; typically referring to the abdomen